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Concentrated Solar Energy Using Nanotubes

One area of research that has attracted a lot of interest deals with ways to concentrate the solar energy received in order to increase the amount of electricity that can be produced.

Miniature antennas made from carbon nanotubes may prove to be the most effective concentration strategy.

Carbon Nanotubes

MIT chemical engineers have found that by using carbon nanotubes (hollow tubes of carbon atoms) solar energy can be concentrated 100 times more than a regular photovoltaic cell.

Such nanotubes could form antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.

Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team and his students tell that their new carbon nanotube antenna, or “solar funnel” might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes.

Nanotube Antenna

Strano’s nanotube antenna boosts the number of photons that can be captured and transforms the light into energy that can be funneled into a solar cell.

The antenna consists of a fibrous column about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes.

Why it Works

Strano’s team built a column made of two layers of nanotubes with different electrical properties - specifically, different bandgaps.

In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.

The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap. That’s important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the centre of the column, where they are concentrated.

Source: The study has been published in the Sept. 12, 2010 online edition of the Nature Materials Journal.